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Police have condemned "violent" protesters who attacked two police officers and set fire to cardboard boxes as they tried to force their way into free trade negotiations.

A handful of police and SkyCity security staff were overwhelmed by more than 150 protesters, forcing the on-the-ground commander to call in reinforcements from around Auckland.

"Police staff moved in to prevent escalation and two officers were separated, attacked and kicked numerous times. Fire appliances were called to the scene to help," police said in a statement. "Two arrests were made. One of these arrests was a female that stomped on a constables head."

One of the protest organisers, Auckland University professor Jane Kelsey, said she had been promised entry to yesterday's meeting to deliver an petition against the Transpacific Partnership Agreement. She was refused entry and instead delivered the document to a representative of the New Zealand negotiation team.

Members of the crowd responded by trying to force their way in. One, Maxine Ngata, she said she was pushed back by a female officer when she tried to enter the hotel.

Among the protesters was Auckland councillor Cathy Casey, who said the trade agreement had implications for everyone. "I was sufficiently concerned to be here today."

Another protester, Green MP Catherine Delahunty, said if the deal was really good for New Zealanders, then the Government wouldn't be so secretive about its contents. "People should not lose their sovereignty because corporates think they can take over the world."

Police said there had been a number of protesters who were genuine in their approach and not violent, but the violent members of their group had put everyone in unnecessary danger.